Megalocytivirus (Iridoviral Diseases of Fish): A Disease Guide

Megalocytiviruses (family Iridoviridae), including RSIV and ISKNV, cause severe systemic disease with anaemia and enlarged cells in many fish. There is no treatment.

Overview

Megalocytivirus is a genus of double-stranded DNA viruses in the family Iridoviridae that cause severe, often highly lethal systemic disease in many marine and freshwater fish. The genus is named for the characteristic enlarged basophilic cells (megalocytes) seen in infected organs. Important members include red sea bream iridovirus (RSIV), infectious spleen and kidney necrosis virus (ISKNV), and turbot reddish body iridovirus (TRBIV). Red sea bream iridoviral disease (RSIVD) is a notifiable disease of the World Organisation for Animal Health (WOAH).

Affected species

Megalocytiviruses infect a wide range of cultured and ornamental fish, mainly in the order Perciformes but also some flatfish and pufferfish. Documented hosts include red sea bream (Pagrus major), groupers (Epinephelus spp.), turbot (Scophthalmus maximus), mandarinfish or Chinese perch (Siniperca chuatsi), and ornamental species such as dwarf gourami (Colisa lalia). Outbreaks have occurred mainly in Southeast Asia and Japan, with detections elsewhere linked to the international ornamental fish trade.

Clinical signs

• Lethargy, loss of appetite, and uncoordinated swimming
• Severe anaemia with pale gills and petechiae of the gills
• Enlargement of the spleen and kidney
• Darkening of the skin and abdominal (coelomic) distension
• Enlarged basophilic inclusion-body cells in spleen, kidney, and other organs (on histology)
• High mortality, often reported in the range of about 20-60% or higher

Transmission

Transmission is thought to occur horizontally when a naive fish ingests tissue from infected fish or is exposed through contaminated water. The movement of live fish, especially through the ornamental fish trade, is an important route for spreading the virus between regions and into previously unaffected populations.

Diagnosis

Diagnosis is based on PCR assays, often targeting the major capsid protein or ATPase genes, supported by histopathology showing the characteristic enlarged basophilic cells, and by immunological methods. Because the gross signs are nonspecific, the enlarged cells and molecular confirmation are key to distinguishing megalocytivirus infection from other systemic diseases.

Treatment, control and prevention

There is no antiviral treatment. Control relies on vaccination where available, biosecurity, and preventing introduction of the virus through infected fish.

• A formalin-killed (inactivated) vaccine is commercially available in Japan for red sea bream and several other susceptible species
• Stock specific-pathogen-free (SPF), tested fish and quarantine new arrivals
• Screen imported and ornamental fish to avoid introducing the virus through trade
• Maintain biosecurity, disinfect equipment, and remove and dispose of infected fish
• Follow WOAH surveillance and reporting requirements for RSIVD